Blood pressure in the human arterial system varies, with the heartbeat, between a maximum or "systolic" value and a minimum or "diastolic" value. Perhaps, the most familiar device for measuring blood pressure is an inflatable cuff or "sphygmomanometer". When the sphygomomanometer is inflated to a pressure between the diastolic and systolic values, the section of artery beneath the cuff collapses and opens with blood pressure variations during the cardiac cycle, collapsing when arterial pressure drops below the cuff pressure and reopening as the arterial pressure increases above the cuff pressure. Should the sphygomanometer be inflated to a pressure which is not between diastolic and systolic, collapse and reopening of the artery does not occur. A meaningful characterization of a patient's blood pressure can therefore be obtained by varying cuff pressure through a range or continuum of values and noting the onset and cessation of arterial activity. The cuff pressure at which arterial activity sets in would then be designated as the diastolic value, and the cuff pressure at which arterial activity ceases would be designated as the systolic value.
Collapse and reopening of an artery produces characteristic "Korotkoff" sounds which can be detected through a stethoscope, ultrasound transducer, or similar device. A familiar procedure for measuring blood pressure has therefore involved sensing the range of cuff pressures over which the Korotkoff sounds can be detected. This procedure requires a skillful operator when performed manually and, when automated, is prone to errors.
Another phenomenon produced by the collapse and reopening of a constricted artery is the production of a pulsatile perturbation in the cuff pressure with each collapse and reopening of the artery. Methods making use of this phenomenon are referred to as "oscillometric". By applying a sequence or continuum of pressures to the cuff and measuring the accompanying pulsatile perturbations, it is possible to obtain a graph of pulse height (or other pulse characteristics, such as a time derivative) versus cuff pressure, which models the patient's blood pressure. Known systems have utilized various techniques for accurate measurement of pulse characteristics (or heights) and storage of the resultant curve which models the patient's blood pressure. For example, U.S. Pat. No. 4,263,918 discloses such a system which measures pulse heights, converts them to digital form and makes use of a microcomputer for processing and storage.
Experiments indicate that the curve which models a patient's blood pressure has a predictable relationship between the pulse heights occurring at various cuff pressures and that this relationship remains consistent over a large population and a variety of absolute arterial pressures. Specifically, the maximum pulsatile perturbation is found to occur at approximately the mean (time-averaged) value of arterial pressure. Moreover, at systolic pressure the perturbation is normally half of that at mean pressure, and at diastolic pressure, the perturbation is approximately 70% of that at mean pressure. These relationships have been found to be substantially stable over time and relatively independent of absolute pressure.
In certain environments, for example, in an operating room, it is necessary to monitor a patient's blood pressure on a constant basis, in order to assure a rapid response to sudden blood pressure changes that might endanger the patient's life or health. In this regard, known oscillometric methods and apparatus for measuring blood pressure have been entirely inadequate. One of the primary reasons for this has been that oscillometric devices require the re-derivation of the full model each time a measurement is provided. This can typically take a minute or longer. During this time the patient could be experiencing a life threatening trauma, and this fact might not be known until the model derivation was complete and a dangerous blood pressure condition indicated.
Broadly, it is an object of the present invention to overcome shortcomings of known oscillometric methods and apparatus for measuring blood pressure which result from the relatively long time required to derive a model of the patient's blood pressure. It is specifically an object of the present invention to achieve a substantial improvement in the speed of detection of changes in the blood pressure of a patient, to the point where a typical detection could be accomplished in about two seconds.
It is yet another object of the present invention to provide a method and apparatus which are readily adaptable to existing automatic blood pressure measuring equipment to achieve a substantial improvement in the speed of detection of changes in blood pressure. It is specifically contemplated that apparatus in accordance with the present invention be capable of being readily retrofitted into existing automatic blood pressure measuring equipment.
It is also an object of the present invention to provide a method and apparatus for rapid measurement of blood pressure which are reliable and convenient in use, yet relatively simple and inexpensive.
In accordance with the present invention, instantaneous changes in a patient's blood pressure are accurately tracked. A characteristic curve is generated so as to approximate the stored blood pressure model in the vicinity of a predefined index value of cuff pressure. The pressure of the cuff is maintained within a predetermined range, r, of said index value, while tracking changes in the index value by sensing a pulsatile perturbation in the cuff pressure, deriving the corresponding value for cuff pressure from the characteristic curve, and utilizing the derived pressure as a new estimate for the index value.